(1) Field of the Invention
The present invention relates to a semiconductor laser, more particularly, to a method of producing a compound semiconductor laser having a multiquantum well (MQW) structure.
(2) Description of the Related Art
Compound semiconductor lasers having various structures have been proposed. Most of these semiconductor lasers are provided with electrodes formed on the bottom surface and the top surface thereof so that a current flows through a semiconductor substrate and epitaxially grown semiconductor layers in a direction perpendicular to the layers. Where an application of a semiconductor laser to an optoelectronic integrated circuit (OEIC) device is considered, it is important to form both the electrodes (i.e., N-electrode and P-electrode) of the laser on the same side of the device, to enable an easy connection thereof to an electronic element. Further, preferably both electrodes of the laser are formed on the top surface thereof (i.e., to form the electrodes on the same plane). As an example of the semiconductor laser having the electrodes on the top surface thereof, a transverse junction stripe (TJS) layer has been proposed (cf. G. H. B. Thompson, `Physics of Semiconductor Laser Devices`, Wiley, 1980, p. 296, FIG. 6.5(b)). In this case, an n-AlGaAs clad layer, an n-GaAs active layer, an n-AlGaAs clad layer, and an n-GaAs contacting layer are epitaxially grown on a semi-insulating substrate, and a P-type region is formed by a diffusion of Zn into the layers and substrate. Both N- and P-electrodes are formed on the GaAs contacting layer, and the GaAs layer is separated by an etched groove. The current flows from the GaAs center layer of the P-type (Zn-diffused) region into the n-GaAs active layer, and light emission occurs in a portion of the GaAs active layer adjoining the PN junction. Since the refractive indices of the clad layers can be made smaller than the refractive index of the active layer sandwiched therebetween, the light can be confined in the vertical direction. On the other hand, in the transverse direction, the refractive index change across a homojunction formed by the p-GaAs (Zn diffused) portion and the n-GaAs active layer is small, so that the confinement of light is not satisfactory.
A semiconductor laser with both electrodes formed on the top surface thereof has been proposed in which the confinement of light in the transverse direction is improved and the carriers are confined in an active region by forming a double-heterostructure in an active layer in the transverse direction. This type of laser is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-134489. In this case, an i-GaAlAs clad layer, an active layer having an MQW structure, and an i-GaAlAs clad layer are epitaxially grown on a semi-insulating GaAs substrate. P-type impurities (Si) and N-type impurities (Zn) are selectively introduced through the active layer from the top surface thereof, to form a P-type conductive region and an N-type conductive region, respectively. The MQW structure of the portions of the active layer in the P-type and N-type region is disordered to give those portions an average composition. Thus the active layer comprises the disordered portions and the remaining MQW structure portion (i.e., the active region) sandwiched therebetween, thus forming a transverse double-heterostructure. The refractive index of the disordered portions is smaller than the average refractive index of the MQW structure active layer. Furthermore, the i-GaAlAs clad layers and the active region of the active layer vertically sandwiched therebetween form the vertical double-heterostructure, and the refractive index of the clad layers is smaller than an average refractive index of an MQW structure consisting of the GaAs wells and GaAlAs barrier layers. Accordingly the current flows through the active region of the active layer from the P-type region to the N-type region in the transverse (width) direction. Accordingly, the light and the carriers are effectively confined in the MQW active region of the active layer, compared with the above-mentioned TJS laser.
The introduction of the impurities for the formation of the P- and N-type regions is performed by an ion-implantation method and a thermal diffusion method. The i-GaAlAs upper clad layer has, in general, a thickness of from 1 to 1.5 .mu.m, to obtain an effective confinement of the light. When the impurities are thermally diffused to extend into the lower clad layer through the upper clad layer and the active layer from the top surface, the impurities diffuse in the vertical direction, and simultaneously, in the transverse direction. This transverse diffusion prevents a narrowing of the width (stripe width) of the active region. Thus the thermal diffusion method cannot attain the formation of a narrow active region having a width of less than 2 .mu.m. The narrow stripe width increases the carrier density, and thus enhances the stimulated emission and reduces the threshold current. On the other hand, when the impurities are ion-implanted so as to reach the lower clad layer through the upper clad layer and the active layer, it is necessary to use an extremely high energy for the impurity ions, which generates crystal defects. Although a conventional annealing for the activation and recovery of crystal defects is performed, unrecovered defects are still present, which generate a current flow that makes no contribution to the radiation. Thus the ion-implantation method does not reduce the threshold current or increase the reliability of the device.